18,000 dinosaur tracks discovered along ancient Bolivian coastline — and they set a new record

Scientists have discovered a record-breaking number of fossilized dinosaur footprints and swim tracks in a national park in central Bolivia.

The tracksite sits along what was once an ancient coastline, with ripple marks extending alongside the footprints and other imprints in a northwest-southeast direction, according to a new study. Most of the tracks belong to bipedal, three-toed dinosaurs known as theropods that lived at the end of the Cretaceous period (145 million to 66 million years ago), but many bird tracks are also preserved, the scientists noted in the paper, which was published Wednesday (Dec. 3) in the journal PLOS One.

"This is the highest number of dinosaur footprints ever found for a single tracksite thus far," study co-author Jeremy McLarty, a paleontologist and associate professor at the Southwestern Adventist University in Texas, told Live Science in an email. "In addition to preserving the most dinosaur tracks worldwide, it also preserves the highest number of swim trackways in the world."

In total, McLarty and his colleagues counted 16,600 theropod footprints and 1,378 swim tracks. These were found in Bolivia's Carreras Pampa tracksite, which was already known but hadn't been properly studied or documented.

Carreras Pampa extends across 80,570 square feet (7,485 square meters) in Torotoro National Park. The initial work involved sweeping debris off the dinosaur imprints with brooms, clearing the tracksite of rocks and removing sediment in places where additional tracks were likely to be found.

The team discovered a huge variety of footprint shapes and sizes, indicating that many types of theropod dinosaurs roamed along the ancient coastline. Several tracks had footprints shorter than 4 inches (10 centimeters), which is rare in the fossil record, according to the study. It's unclear if these footprints were made by small theropod species such as Coelophysis or by juveniles of larger species, the researchers wrote.

The largest footprints were more than 12 inches (30 cm) long, and the team thinks these may have been made by mid-size theropod dinosaurs such as Dilophosaurus or Allosaurus. Large theropods such as Tyrannosaurus rex and Giganotosaurus typically leave 16-inch-long (40 cm) footprints, the researchers noted.

Carreras Pampa is unique because the footprints show different dinosaur behaviors, such as walking, running, swimming, tail-dragging and making sharp turns. "It preserves evidence of several types of unusually preserved locomotive behaviors, and preserves one of the highest numbers of dinosaur tail traces anywhere in the world," McLarty said.

The swim tracks are straight or comma-shaped grooves that often have one or two similar but smaller grooves next to them, McLarty said. The main groove is from theropods scratching the sediment at the bottom of the water with their middle toe, while the smaller grooves are from the other toes. Unlike other sites that preserve only individual dinosaur swim tracks, Carreras Pampa preserves alternating left and right tracks, he said.

The abundance of imprints shows that Carreras Pampa was a prehistoric highway, and the parallel orientation of some trackways suggests some dinosaurs traveled in groups.

Bolivia is known for being a dino track hotspot.

"The tracksite with the next highest number of tracks is also in Bolivia," McLarty said. "The Cal Orck'o tracksite is located in an active quarry as a nearly vertical wall and is long and thin. The Carreras Pampa tracksite is spread out across a wider area."